1. Field of the Invention
This invention relates to a magnetic head actuator for causing a magnetic head to perform a tracking action by displacing the magnetic head in an apparatus such as a VTR (magnetic tape recorder) or the like.
2. Description of the Related Art
The known device of this kind has heretofore been arranged to use a voice coil or a bimorph element.
FIG. 1 shows by way of example the magnetic head actuator using a voice coil. The illustration which is a sectional view taken in the direction of the axis of a cylindrical shape includes cylindrical permanent magnets 201a and 201b; an inner yoke 202; outer yokes 203a, 203b and 203c; magnetic heads 208a and 208b; gimbal springs 205 and 206; a coil bobbin 204; and the voice coil 214.
The two cylindrical permanent magnets 201a and 201b are arranged to have their same magnetic poles (N poles, in this case) opposed to each other. Therefore, magnetic fluxes emitted from these magnets 201a and 201b pass the inner yoke 202 and an air gap part 210 where the voice coil 214 is disposed and come back to the S poles of the magnets 201a and 201b through the outer yokes 203a, 203b and 203c. A radial magnetic field is thus formed within the air gap part 210 from the inner yoke 202 toward the outer yoke 203b. The voice coil 214 is mounted on the two gimbal springs 205 and 206 which are disposed outside the permanent magnets 201a and 201b. The voice coil 214 is thus supported so as to be displaceable solely in the axial direction thereof. With a current applied to the voice coil 214, therefore, an electromagnetic force is generated between the voice coil 214 and the magnetic field within the air gap part 210. The voice coil 214 thus can be axially displaced by the electromagnetic force up to a position where the voice coil 214 comes to be balanced with the reaction forces of the gimbal springs 205 and 206. Referring to FIG. 2, the magnetic heads 208a and 208b are secured to a projection formed in a part of the gimbal spring 206 which is secured to the voice coil 214. The magnetic head actuator is arranged in this manner.
According to the above-stated arrangement, however, the outside diameter of the magnetic head actuator becomes larger as the gimbal springs 205 and 206 are disposed on the outside of the diameter of the air gap part 210.
Generally, the performance of the magnetic head actuator (in terms of amplitude, response frequency, etc.) increases with the volume of the air gap part and the magnetic flux density. In other words, the performance degrades accordingly as the sectional area of the magnet decreases. According to the conventional arrangement, therefore, the diameter of the magnetic head actuator is difficult to reduce to a sufficient extent in conformity to the reduction in diameter of a rotary head drum as desired for reduction in size of a VTR.
In the case of a digital VTR or the like, the rotational frequency of the rotary head drum has been increased. Therefore, it is difficult to improve the response frequency of the magnetic head actuator.
Another problem with the conventional device lies in the adverse effect of a leakage flux resulting from having the magnetic-head mounting part disposed close to a cutout part provided in the outer yoke.
A further problem with the conventional device lies in the following point: In a case where two or more magnetic heads are to be mounted, the size of the cutout part increases to make the rigidity of the device insufficient because of the increase in number of the heads including a wiring space necessary for them.